In a former serial dot impact printer (hereinafter: printer), if print density of print operation is high, load with respect to electric source is extremely large, and the droop of electric source voltage is easy to occur. These days in which low price of printer is advancing, it is difficult to realize to develop an electric source apparatus in which the droop of electric source voltage does not occur even though a high-density print is executed, and which does not influence the print at all even though the electric voltage droops, because cost of the electric source apparatus is raised and causes high price of printer.
According to such situation, the former printer furnishes mechanism that can detect the droop of the electric source voltage, and printer control program further detects the droop of the electric source voltage by using electric source voltage droop detecting mechanism, when the droop of the electric source voltage is detected, low speed print in which the print speed is reduced and division print in which one line is printed by moving the print head plural times through plural paths print are performed, and the load reduction to the electric source apparatus of the high-density print operation time is attempted.
Further, there is such one that prints in a low voltage in the case of the high-density print; and prints in a high voltage in the case of the low-density print (for example, referring to patent document 1).    Patent document 1; Japan patent publication of No. 11-58841 (Paragraph [0029]˜Paragraph [0030], FIG. 1 and FIG. 8)
However, in the former technology stated above, when the electric source voltage droop is detected through print data whose print density is high, because low speed print and division print are executed and print time becomes long, there is a problem that print efficiency is low. Especially, even when instantaneous drop of the electric source voltage and the electric source voltage droop through print data whose print density is high in a short region that exists in a print block are detected, because the low speed print and the division print are executed, there is a problem that the print time becomes long.
For example, as shown by FIG. 16, in print block in which there are print data whose print density is high on the left side of the figure (high-density print block) and print data whose print density is low on the right side (low-density print block), in the case to print through moving the print head from the left side to the right side of the figure, the droop of the electric source voltage is started from a position shown by P1 in the figure through the print of the high-density print block. The control program of the printer, through detecting the droop of the electric source voltage, executes a low speed print control that gradually reduces print speed. The low speed print control that reduces the print speed is continued until the electric source voltage becomes low to “division print start voltage” that is shown by P2 in FIG. 16. Furthermore, when the electric source voltage droops to the division print start voltage point shown by P2 in the figure through continuing the print operation, a path division print control is started through plural paths print of over two paths from the detection point. The path division print control is continued until print of block that is being printed is completed.
Thus, when the droop of the electric source voltage is detected, because the low speed print and the division print are executed and the print time becomes long, the print efficiency becomes low.